Igneous electrolysis cell



April 22, 1952 1 GROLE 2,593,751

IGNEOUS ELECTROLYSIS CELL Filed May 17, 1948 2 SHEETS-SHEET 1 FeQl 4 W W y /,f iff \x INVENTQR. Je an @role e BY m April 22 1952 J. GROLE 2,593,751"

IGNEOUS ELECTROLYSIS CELL Filed May 17, 194e 2 sHEETs-SHEET 2 INVENTOR.

l J ean Groleg Aorney Patented Apr. 22, i952 IGNEOUSLELECTROLYSSIS CELL Jean Grole, Paris, France, assignor to Pechiney- `Compagnie de Produits Chimiques et Electrometallurgques,v a corporation of France Application May 17, 1948, Serial No. 27,568 In France September 5, 1947 (Cl. 21M- 243) 4 Claims.

,l 4 Igneous electrolysis cells work with a voltage which averages from 4 to 6 volts. It is, therefore, of interest to reduce as much as possible the voltage drops that are not absolutely necessary to the electrolytic operation. This is the case of the voltage drop in the bottom of the Crucible which constitutes, for instance, the cathode portionof the cell in the electrolytic treatment of alumina dissolved in cryolite in the production of aluminum. On the contrary, this portion constitutes the anode portion of the cell used to rene the aluminum. It is also of interest to mainE tain this voltage drop at a low value during the whole life of the lining of the electrolytic cell and to make this lining last as long as it is possible, sofas to keep the electrolytic Cell in operation with theminimum of interruption. The lining of the Crucible that contains the electrolytic bath is made of Carbon;v the bottom that constitutes the Cathodic Vportion of the electrolytic cell is often built of carbon blocks that have been subjected to a preliminary baking treatment, and the voltage drop resulting from the flow of the electrolytic current through the bottom depends essentially upon the electric connection between these blocks and the current conveying metallic bars, which are generally made of steel or copper.

Thiselectric connection is generally ensured by cutting in the block of agglomerated carbon a groove .the transverse dimensions of which are greater than those of the current leading bar. This bar is then inserted in the groove and the space left between the bar and the agglomerated Carbon mass is packed with a hotpaste constituted by a mixture of Carbon dust and tar pitch or any other carbon containing product, such as molasses (sugar paste). `When the electrolytic cell is started into operation, this paste is baked and thus ensures the electric connection between the agglomerated carbon block and the current leading metal bar.

4Voltage drops in the bottom of electrolytic cells constructed in the above indicated manner range commonly from 0,45 volt to 0.50 volt when the cell is new, and may become as high as 0.80 volt after a time.

In order to improve this voltage drop, it has been suggested to ensure the electric connection between the agglomerated carbon mass and the metallic bar by means of cast iron. In this way a better voltage drop was obtained in the bottom of the cell when the latter was new, however, some carbon blocks .crackedwhen molten cast viron was poured thereon and they had to be replaced, Which increased the expenses. On the other hand, a great'number of carbon blocks, instead of breaking completely, suffered slight cracks, which grew bigger and bigger during operation of the electrolytic or rening cell.

These cracks produced an increase in the voltage drop at the bottom of the cell because the partly cracked carbon black offered an abnormal resistance to the ow of current. Furthermore, these small cracks limited the life of the lining of the :bottom of these cells, as will be explained more in detail hereinafter.

y These serious drawbacks have made it impossible to develop the process consisting in the pouring of molten Cast iron on the metal bars to seal them in the carbon blocks.

The object of the present invention is to proa vide a sealing of the current leading metal bars which avoids these drawbacks.

For this purpose, according to an essential feature of our invention, the sealing of the current leading metal bars in the agglomerated carbon blocks is ensured by means of a Cast metal having a melting temperature above l000 C. and which, after solidification, is substantially free from components susceptible of undergoing a transformation, with increase of volume, when kept for a long time at a temperature of 900 C.

Indeed, in electrolytic Cells constructed in this manner, inasmuch as the voltage drop in the bottom of the Crucible averages 0.25 volt, this voltage drop is insufficient for producing through the joule eiect in the pot bottom an amount of heat sufcient for keeping it at the desired tem-` perature. It is absolutely necessary to heat insulate this bottom in order to avoid the depositing of a bath in the pasty, and even solid state on the upper portion of the carbon blocks, for such a deposit would increase the voltage drop so that the desired gain on the total voltage of the electrolytic cell would be neutralized by this increase. Due to this heat insulation, the lower portion of the agglomerated carbon blocks, where the current leading metallic Vbars are sealed in, is maintained, under normal working conditions, at a temperature of about 900 C. Now, it has been found that, under these conditions, the cast iron of ordinary quality, that was used for sealing purposes, was subjected to an increase of volume much larger than that due to its normal expansion by reason of the rise of temperature. This swelling of the sealing cast iron widened the Cracks'in the carbon blocks and produced the serious accidents above referred to. In some cases,v vswelling tooky lplace to such a degreev that it Caused the cracking of carbon blocks which 3 had remained in perfectly good form during the building of the lining of the bottom of the electrolytic cell. u

These drawbacks are fully avoided by making use of a metal such as, for instance, copper for sealing the electrode leading bars. Brass containing less than 10 per cent of tin is also well adaptedfor this purpose. The same is also true of cast iron free from sulfur and combined carbon. But any metal or alloy having a melting point higher than 1000" C. and which,` after solidication, contains no component liable to undergo a transformation with an increase of volume by the prolonged application of a temperature of 900, can also be used for this purpose.

A cast iron having the following composition gives good results when used for the sealing of the current leading metal bars: Y

Per cent Carbon approximately 3 Silicon from 2.5 to 3 Phosphorus from 1 to 1.5 Manganese less than 0.5 Sulphur less than 0.05

Preferred embodiments of my invention will be hereinafter described with reference to the accompanying drawings, given merely by way of example, and in which:

Fig. l is a longitudinal vertical section of the electrolytic cell;

Fig. 2 is a corresponding transverse vertical section;

Figures 3, 4 and 5 are views showing different ways of sealing the current leading bars according to the present invention;

Fig. 6 is a perspective view showing three carbon blocks ready to be sealed with the same metal bar for current feed;

Fig. 7 is a plan view illustrating the sealing of a current leading metal bar in a carbon block of elongated shape;

Fig. 8 is an explanatory view illustrating a method of pouring the sealing cast iron in three steps to obtain a sealing as shown by Fig. 4;

In Figs. 1 and 2 of the drawings, reference numeral l designates the metallic casing in which is built the electrolytic cell lining. The casing I is insulated from the carbon lining 3, 4 and current leading metal bars by means'of refractory bricks 2. The carbon blocks 4, which constitute the lining of the bottom of the electrolytic cell, are connected with current leading metal bars 5 disposed in grooves 1, cut in the carbon blocks, by means of the cast iron portions 6.

In the known devices, the current leading metal bar is disposed in the groove cut in the carbon block and liquid cast iron is poured in the spaces between said metal bar and the carbon block. But when the block in question is in position at the bottom of the crucible, solidified cast iron does not project from the under horizontal face of the carbon block.

As above mentioned, with this sealing method the voltage drop was lower than in the absence of cast iron poured between the metallic bar and the carbon block, however a considerable number of blocks cracked during the pouring of the metal and it was necessary to replace them. On the other hand, a great number of carbon blocks, without being thoroughly cracked, suffered from vslight cracks which opened more and more during the working of the electrolytic or refining vcell. These small cracks involved an increase f of the voltage drop in the bottom of the electrolytic cell. because the partly cracked carbon block had an abnormal resistance to the flow of electric current. Furthermore, thesesmall cracks limited the life of this bottom lining of the electrolytic cell. For, when a crack occurred on either side of the cast iron seal, this small crack, widened by the abnormal swelling of cast iron, slowly spread toward the edges of the carbon block and the upper portion of this carbon block finally broke off. The liquid located at the upper portion of the carbon blocks, constituted, for instance, of molten aluminum, got soiled by contact with the sealing cast iron and the metal bar, which made it necessary to stop the operation of the electrolytic cell in order to repair the crucible bottom. The same accident occurred, although less suddenly, when a single crack was produced in the carbon block 4, starting from the cast iron seal 6 and extending upwardly as a result of the pouring of molten cast iron into the space between bar 5 and carbon block 4. The crack spread gradually inthe upward direction, and liquid aluminum filtered through this crack into contact with the cast iron seal.

These drawbacks are avoided by making use, according to an essential feature of the present invention, for sealing of the current leading metal bars in the agglomerated carbon blocks, of a molten metal having a melting point higher than l000 C. and which, after solidiilcation thereof, is substantially free from components liable to undergo a transformation with an increase of volume when kept for a long time ata temperature of 900 C.

In the arrangement illustrated by Fig. 3, which is in accordance with the present invention, .cast iron E, projecting from the sealing groove 1, forms a coating 6a, from 1 to 2 cms. thick, along the lower horizontal face of carbon block 4.

It is clear that when the metal bar is sealed in position, shrinkage of cast iron in groove 1 exerts on portion 6a a force which tends to apply it firmly against the block, which is favorable from an' electrical point of view. In this way, it is possible to obtain a voltage drop averaging 0.20 volt.

This gain is further increased by making use of a form of seal such as shown by Figs. 4 and 5, according to which the cast iron sealing portion 6 not only lls groove 'I and covers the lower horizontal face of the block at Ba, but also surrounds said block on the four vertical faces thereof, at 6b, up to a height which may reach l0 cms. The voltage drop can thus be reduced to 0.14 or 0.15 volt.

The current leading metal bars 5 are generally made of soft Martin steel, of forging quality, having a breaking strength of 45 kgs. and an elongation of 30 per cent.

They are straightened in a press. This is necessary, when they are constituted of elements of square section, to avoid any warping thereof, as otherwise, after the cast iron seal is poured, warped expansion thereof will exert detrimental effects -on the good quality of the joint and may even produce cracking of the carbon blocks.

To further enhance the good construction of the crucible bottom, the admissible sag on one of the kfaces should not exceed 1.5 mm. per meter, but this condition is not necessary for the obtainment of a good seal in itself.

The portion of the conductor bar. made of iron or copper, which is to be in contact with the sealing metal, should be sand blasted with dry sand and air, or preferably ground, so as to remove any trace of oxide thereon. It must be neither moist nor greasy at the time of the sealing operation. On the other hand, there is no disadvantage if it has rough surfaces and retains the marks of the tools that have been used thereon in case it has been machined (which is not necessary). l

I have found that it is advantageous to heat the carbon blocks beforethe .sealing operation is performed. The best way of operating is to place them in an oven so that they are heated uniformly on all their faces. It may be sufficient to heat only the face that is grooved For carbon blocks of cubic shape of a size of 50 cms., a temperature'of 80 C. in the surface zones is considered as sufficient.

Preheating of the carbon blocks is not an absolutely necessary condition and skilled workmen can make a successful job when sealing the carbon blocks in the cold state provided that they are quite free from moisture. However, this heating step is advantageous.

The melting point of the particular cast iron above defined is about 1150 C.

However, it is well to heat it to a temperature such that, after pouring into the casting ladles and transfer of the latter to the place Where the sealing operation is to be performed, the temperature of the molten metal ranges from 1200 to 1300" C. This makes it possible to obtain the fluidity necessary for a good sealing operation.

However, it is unnecessary and even dangerous to heat the metal to a much higher temperature, as this would increase the already very intense thermal shock undergone by the carbon block.

Obviously, the liquid cast iron is to be thoroughly cleaned and, indeed several times before pouring.

It may be sufcient to dry the current leading metallic bars in ovens at a moderate temperature, but it is advisable not to heat them too much as,'otherwise, oxidation might occur, which, as above stated, is to be carefully avoided.

Fig. 6 shows, by way of example, an arrangement which, is advantageous to use for the sealing of a metallic vbar in three carbon blocks of 500-600 mm. length.

The carbon blocks 4, placed on the ground, with the sealing groove 'I turned upwardly, are alined, if necessary, by means of any suitable means, so as to permit the insertion of bar in position. The ends of this bar are placed on two supports 8 which hold it at a level such that it extends through all the grooves T of the blocks. An important requirement is that the unsealed ends of bar 5 must be capable of expanding freely. Consequently, it is preferable not to fix them by means of bolts, `wedges or keys.

In the case of horizontal bars engaged in the grooves 1 of several carbon blocks 4, I provide, between said blocks. the necessary `oints 9 for causing the cast iron to ll up the groove 1 of every block, but not to overflow from one groove to that of the next block. As a matter of fact, the problem is to provide expansion joints in the cast iron mass. The carbon blocks are connected together by means of a molding frame I0.

The pouring operation is performed with the precautions above stated concerning temperatures and cleaning of the metal. If there are three carbon blocks'on the same bar 5, the three blocks 4 are simultaneously sealed by bringing into action three separate teams of workmen.

It is necessary to proceed in such .a manner 'that the jet of liquid metal .does not strike directly the bar and never stays at the same sealing point, but on the contrary, is caused to travel in the spacebetween the carbon block and the metal bar, in such manner as to avoid dangerous local heating. As soon as the sealing surface starts solidifying, everything that might oppose a quick cooling is immediately removed (for instance, the asbestos and sand joints 9, the moulding frame I0, which is to be immediately loosened, and so on). This is done because it is necessary to` remove heat as quickly as possible into the surrounding atmosphere, so as to reduce as much as possible the absorption thereof by the carbon blocks and thus avoid cracking thereof.

vIn order to avoid any kind of strains, which might generate internal stresses, it is advisable to let expansion take place freely, not only in the metallic bar, but also in the carbon blocks themselves, which are to be surrounded, for inl stance by a fixed moulding frame, for only just the time strictly necessary for the pouring operation. f

A last precaution consists in avoiding transportation and displacements of the assembly formed by the carbon blocks and bars sealed therein before suitable cooling thereof. It is necessary to wait for several hours after the sealing operation, and preferably, till the next day.

In order to obtain a seal as disclosed by Fig. 4, it has been found advantageous to proceed in three steps, as indicated by Fig. 8.

In order to form the rst portion Il of the seal, groove 1 is filled up to the level of the horizontal face of the block, this being done at a suitable place, as explained with reference to Fig. 6. The second portion I2 of the seal is poured on the spot, that is to say in the cell, the refractory lining under the carbon blocks acting as a mould. The third portion I3 is of course formed on the spot. Fig. 8 shows the arrangement of a cathode assembly for an aluminurn furnace, with a seal of the kind disclosed by Fig. 4. The cast iron portions Il, l2, |3 of the block weld together in a sufficient manner at the place of the joints.

It should be well understood that the successsive sealing operations take place with time intervals sufficient for keeping the whole of the blocks and barsr at a temperature not too different `from that of the surrounding atmosphere. However, it has been possible to cast simultaneously (in advance) portions H and I2, by making use of a mould for portion I2.

The method above described is satisfactory for sealing current leading metallic bars in carbon blocks the size of which does not exceed 600 mm.

For a long time, it was considered as unpractical to seal by means of cast iron or brass, in carbon blocks constituting the lining of the bottom of igneous electrolytic cells or electrometallurgical furnaces, the current conducting metallic bars, when the seal exceeded a length averaging 500 or 600 mm.

This was due to the fact that the first experiments had been very unsatisfactory, as the carbon blocks cracked or broke and became useless, while, due to their dimensions, they were Very expensive.

The method according to the present inven- 'tion has. made it possible to seal safely long blocks on horizontal bars, and of constructingpot bottoms having the same characteristics of long life and 10W voltage drop as those obtained with shorter blocks.

As a matter of fact, it was found that the unsatisfactory results experienced prior to my invention were due to the fact that sufficient precautions were not taken to reduce the detrimental effects of the thermal shock on the carbon blocks and of the expansions or contractions of the bar or of the cast iron portion.

In order to reduce the strains they create in the carbon blocks, it has been found that it was advisable:

To cast the sealing portion section by section; To provide a free space of about 2 or 3 cms. of width between the sections;

To pour these sections at given time intervals and in a given order;

To regulate the removal of heat;

To give a regular profile and relatively smooth walls `to the lateral faces of the current conducting bar housings.

I will now proceed to give, by way of example, the description of the sealing of a carbon block of a size of 1500 X 500 x 500 mm. (Fig. '7).

All the steps preparatory to the sealing operation are the same as in theconventional method, that is to say pre-heating of the block, cleaning of the bar, melting of cast iron of definite (i. e. well determined) quality.

Carbon block 4, turned upside down so as to have its groove 1 turned-upwardly, is fitted with bar 5 fixed in proper position with respect to the groove. The space into which cast iron will be poured is divided into three portions, through suitable means. |lhis division into three portions, with expansion joints I4, permits of delaying the transmission of heat from one element to the other of the seal and gives the cast iron element possibilities of expansion when it is subsequently heated, in service, to a temperature averaging 900 C'.

Each of the sections of the seal is poured separately, with a time interval of several hours, so that the amount of heat supplied by the liquid cast iron that is poured is fed in three separate steps, a circumstance which is advantageous to ensure a good resistance of the carbon block and to permit expansion of the bar.

The order in which the respective sections are cast is not immaterial. It has been found that it is much more advantageous to proceed first to the sealing of the middle portion I5. For it goes without saying that the thermal interchanges and the expansion stresses are then most favorable.

As soon as the metal isv solidified, the joint that separated the seal that has just been cast from the next one is destroyed, in order to facilitate cooling, which cooling may be intensified by circulating a slight air stream through the space thus cleared.

About one hour is allowed to elapse, and it is advisable to protect by means of heat insulated covers the sections that have`not yet been sealed, so that they are slowly heated throughout their mass.

After some time, further means are applied for protecting the carbon block against too quick a cooling, by covering the portions that are still uncovered. These precautions are taken solely because it is desired to have a hot block, that is to say a block at a temperature ranging from 40 to 80 C. when the casting of the next section is to take plac. This procedure ensures that there is no moisture resulting from condensation and slightly reduces the thermal shock andthe internal strain that will result from the next step of the process.

If the length of the carbon block makes it necessary to'use four sections, it is a good thing always to seal first the intermediate sections, so `as to avoid leaving an appreciable portion of bare metal bar (for instance 40-50 mm.) caught between two sealing lsections already formed, or one of which is being formed. For reasons relating to expansion requirements, this way of proceeding is to be avoided.

Concerning time considerations, it is advantageous to leave a time interval of twelve hours and, if possible, twenty-four hours between the casting of two consecutive sealing sections.

In a general manner, too hasty a casting of the respective elements (even if occasionally it gives a good result) is not safe and involves probabilities of having a lot of elements cracked and broken, while, normally, practically no breaking should occur.

It is preferable not to seal both ends' simultaneously.

As a supplementary safety, a last precaution,

of lesser importance, may be mentioned. It is advantageous to have the profile of the lateral facesof the groove 1 provided in the carbon block as even as possible, and these faces should be relatively smooth so as not to oppose an undue resistance to the slight movement of displacement of solidied metal masses or in course of solidication. Wheni the above described precautions are complied with, the agglomerated carbon blocks are perfectly sound and without cracks after the current leading metal bars have been sealed by means of molten metal. I then obtain a voltage drop in the lower portion of the retort which. when the cell is new, averages 0.25 volt for a current density in the carbon blocks of about 0.5 ampere per square centimeter. This portion of the retort may last for four and even five years, the voltage drop then becoming 0.35 and at most 0.40 volt.

The certainty of obtaining, after sealing of the bars, carbon blocks which are perfectly sound permits of still improving the voltage drop at the contact of the current leading bars with the carbon blocks by taking advantage of the contraction of the metal that surrounds the block when this metal is cooled. Such an arrangement, illustrated by Figs. 3, 4 and 5 permits of reducing the voltage drop down to 0.15 volt.

The ow of current through the bottom of an electrolytic cell made as described in the preceding paragraph causes a voltage drop ranging from 0.15 to 0.25 volt. The heat given oi by the joule effect is not suiiicient to keep the pot bottom at its working temperature in the absence of heat insulating means. It is therefore necessary to provide, under the bottom and on the lining of the sides of the electrolytic cell, layers of refractory and heat insulating bricks, the thickness of which depends upon the Voltage drop in the bottom of the pot.

By way of example, for a voltage drop of0.15 volt, good results are obtained by providing under the carbon lining of the bottom a layer of 50 cms. of refractory bricks under which is a layer of 40 centimeters of insulating bricks. On the lining of the sides of the pot, the thickness of refractory material varies, from top to bottom, from 15 to 40 cms.

For a voltage drop of G25-0.30 volt, it is sufcient to provide, under the carbon lining of the Ibottom, a layer of 20 cms. of refractory bricks above a layer of 13 cms. of insulating bricks.

In a general manner, while I have, in the above description, disclosed what I deem to be practical and eiiicient embodiments of my invention, it should be well understood that I do not wish to be limited thereto as there might be changes made therein without departing from the principle of my invention as comprehended Within the scope of the appended claims.

What I claim is:

1. An igneous electrolytic cell comprising: an inner lining for the bottom portion thereof formed of baked carbonaceous material, current conducting metal bars positioned in said lining, and a cast seal of low electrical resistance between said lining and said bars consisting of cast iron containing approximately 3% carbon, 2.5 to 3% silicon, 1 to 1.5% phosphorous, less than 0.5% manganese, less than 0.05% sulfur, and the remainder iron, the material of said seal being characterized in having a melting point in excess of 1000 C., and in being substantially free, fol lowing solidication, of constituents susceptible of undergoing transformation with increase in volume when subjected to temperatures in the range of 900 C. for prolonged periods of time, whereby the production of destructive stresses in the lining during the formation of the seal and during the operation of the cell is avoided.

2. A cell in accordance With claim 1 wherein the carbonaceous lining is provided with an insulating jacket spaced from the lower horizontal surface of the lining, and the material of the seal projects below the lower horizontal surface of the carbonaceous lining and forms integral lateral flanges in close contact with the lower surface of the lining, whereby said flanges are tightly pressed against said lining upon solidication of the seal.

3. An igneous electrolytic cell comprising: an inner lining for the bottom portion thereof formed of baked carbonaceous material, current conducting metal bars positioned in said lining, a cast seal of low electrical resistance between said lining and said bars consisting of cast iron containing approximately 3% carbon, 2.5 to 3% silicon, 1 to 1.5% phosphorus, less than 0.5% manganese, less than 0.05% sulfur, and the remainder iron; the material of said seal being characterized in having a melting point in excess of 1000 C., and in being substantially free, following solidiflcation, of constituents susceptible of undergoing transformation with increase in volume when subjected to temperatures in the range of 900 C. for prolonged periods of time, whereby the production of destructive stresses in the lining during the formation of the seal and during the operation of the cell is avoided; a housing for said cell, the lower horizontal surface of said lining being spaced from the adjacent portion of the housing and the material of the seal projecting below the lower horizontal surface of the carbonaceous lining and forming integral lateral flanges in close contact with the lower surface of the lining, whereby said flanges are tightly pressed against said lining upon solidication of the seal; said lining being formed of spaced blocks and the said flanges extend both over the lower surface of the blocks and the vertical sides thereof.

4. A cell according to claim 3, characterized in that the lining is provided with a heat insulating jacket.

JEAN GROLE.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 8,658 Osgood Jan. 13, 1852 119,682 Yielding Oct. 3, 1871 450,105 Le Sueur Apr. '7, 1891 1,279,192 Wheeler Sept. 17, 1918 1,701,656 Artema et al. Feb. 12, 1929 2,353,444 Conradty et al July 11, 1944 2,378,142 Hurter June 12, 1945 2,388,123 Conradty Oct. 30, 1945 2,390,805 Merryman et al. Dec. 11, 1945 FOREIGN PATENTS Number Country Date 72,487 Norway Aug. 25, 1947 OTHER REFERENCES Handbook of Chemistry and Physics, 28th ed., 1944, Chemical Rubber Publishing Co., Cleveland, Ohio, page 1217.

Alien Property Custodian publication No, 334,093, June 1, 1943. 

1. AN IGNEOUS ELECTROLYTIC CELL COMPRISING: AN INNER LINING FOR THE BOTTOM PORTION THEREOF FORMED OF BAKED CARBONACEOUS MATERIAL, CURRENT CONDUCTING METAL BARS POSITIONED IN SAID LINING AND A CAST SEAL OF LOW ELECTRICAL RESISTANCE BETWEEN SAID LINING AND SAID BARS CONSISTING OF CAST IRON CONTAINING APPROXIMATELY 3% CARBON, 2.5 TO 3% SILICON, 1 TO 1.5% PHOSPHOROUS, LESS THAN 0.5% MANAGANESE, LESS THAN 0.05% SULFUR, AND THE REMAINDER IRON, THE MATERIAL OF SAID SEAL BEING CHARACTERIZED IN HAVING A MELTING POINT IN EXCESS 